Heat treatment of grain oriented steel to obtain a substantially constant magnetic permeability



Dec. 22, 1964 P. s. FRISCHMANN ETAL 3,152,554

HEAT TREATMENT OF GRAIN ORIENTED STEEL TO OBTAIN A SUBSTANTIALLY CONSTANT MAGNETIC PERMEABILITY Original Filed Oct. 5, 195

Fig.2. f A

F-{gi f A A In ve rvror's:

Peter- G. Frischmann,

Jahrv L.

United States Patent 2 Claims. (Cl. 148-113) This application is a division of applicants copending application S.N. 60,720, filed October 5, 1960, now Patent No. 3,108,912 entitled Magnetic Material, and assigned to the same assignee as the present invention.

This invention relates to bodies of magnetic alloys for use in induction apparatus and more particularly to high purity silicon iron alloys having unique magnetic properties and to a process for producing these bodies.

In the construction of the many types of induction apparatus, magnetic materials having diverse and often special magnetic properties are required. The applications in which any given magnetic material may be used depend upon such properties as the coercive force, permeability and residual induction. Heretofore, the construction and operation of certain types of communications equipment, for example, have required the use of special alloys or specially constructed bodies to achieve essential magnetic properties. Specifically, for such uses as filter coils in radio and television circuits and as loading coils for long distance telephone transmission lines, the permeability should be substantially constant and the eddy current losses low, particularly at low field strengths, to obtain true reproduction of sound. Early attempts to attain constant permeability involved the use of concentrated air gaps in the core, but these attempts were generally discontinued due to the effects of flux leakage upon the surrounding circuitry. Although the permeability of ferromagnetic materials is inherently not constant, fair success in obtaining constant permeability was achieved by using compressed iron powder. The iron powder particles were used both insulated and uninsulated, the former giving somewhat lower values of permeability than the latter and also having much lower eddy current losses. The permeabilities of these materials ranged from about 25 to 75 at inductions (B) of up to 100 gausses.

Higher values of constant permeability of up to 2,000 at inductions of up to 1,000 gausses have been achieved in special types of relatively expensive high alloy metals. For example, the Perminvars, which are alloys usually containing from about 10 to 55 weight percent nickel, 15 to 60 weight percent cobalt, remainder substantially all iron, have substantially constant permeability but the particular metals used render them comparatively expensive. The Perminvar composition ranges listed are not intended as limiting since other proportions of the constituent metals may deliver suitable properties for some uses. related alloys can be obtained from publications such as Ferromagnetism, by Richard M. Bozorth, published in 195 6 by D. Van Nostrand Company, Inc.

A principal object of this invention is to provide bodies of silicon-iron alloys having substantially constant magnetic permeability.

Another object of this invention is to provide ironbase bodies containing from 4 to 6 weight percent silicon which have substantially constant permcabilities in low magnetic fields.

A further object of this invention is to provide siliconiron alloy bodies oriented in the (110) [001] crystalline Further information regarding Perminvars and.

orientation, which bodies have substantially constant mag netic permeability.

An additional object of this invention is to provide a process for producing silicon-iron alloy bodies having substantially constant magnetic permeabilities.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

In the drawings:

FIG. 1 shows hysteresis curves for oriented silicon-iron alloy bodies used to produce bodies according to the present invention;

FIG. 2 shows hysteresis curves of the unique bodies of this invention; and

FIG. 3 shows the hysteresis curves of the bodies of FIG. 2 following additional treatment.

Generally, the bodies of the present invention are made of high-purity iron-base alloys containing from 4 to 6 weight percent silicon and not more than about 0.01 Weight percent incidental impurities. These bodies, when processed according to the method of the invention, have substantially constant magnetic permeabilities. The novel process of this invention comprises preparing a highpurity alloy of the composition previously mentioned and processing it through a plurality of hot and cold rolling stages with intermediate anneals to develop a strong [001] crystalline orientation. The oriented bodies are then given a final anneal in selected atmospheres to produce the constant permeability.

Considering the processing in more detail, cast ingots are prepared by vacuum melting high-purity iron and silicon together in proportions such that the silicon content of the final alloy falls between 4 to 6 percent. The impurity content of the cast alloy should not exceed 0.010 weight percent as a maximum and should preferably not exceed more than about 0.006 weight percent. The normal content for some of the more usual'ir'npurities is carbon, 0.001; sulfur, 0.001; oxygen, 0.001; and nitrogen, 0.0005; the remainder being minor metallic impurities which are normally present.

Once a cast ingot is obtained, it is hot rolled to 0.25 inch. At this point, the material is annealed at 700 C. to 1200 C. for from about 0.1 to 1.0 hour in dry hydrogen, that is, hydrogen which is substantially non-oxidizing to silicon, viz., dew point no higher than 40 F. The slabs are then rolled at from 700 to 1000 C. to about 0.100 inch thickness. The material at this stage is subjected to an annealing at temperatures ranging from about 700 C. to 1200 C. for a time sufiicient to recrystallize the hot-worked texture, e.g., about 0.1 to 10 hours in a substantially nonoxidizing atmosphere such as dry hydrogen. It is to be understood that all reference to dry hydrogen means hydrogen having a dew point no higher than 40 F., unless specified to the contrary.

The annealed product is then cold Worked at least 25 percent in substantially the same direction (e.g., over the range of 40 percent to 99.5 percent) using intermediate anneals where required when more than one reduction stage is used to arrive at thicknesses ranging up to about 0.015 inch. The intermediate anneal which is also carried out under substantially nonoxidizing conditions may range in temperature from about 700 C. to 1200 C. for a time sufiicient to effect recrystallization, such as 0.1 hour to 10 hours.

After the final thickness has been reached, the coldworked material is then subjected to an anneal at an elevated temperature in a selected environment for a sufficient time to effect secondary recrystallization. Temperatures for the final anneal may range from about 1000 C. to 1350 C. for times ranging up to about 16 hours for thicknesses ranging up to about 0.015 inch, the annealing time increasing with increasing thickness and de- 11 shown in FIG. 1 of the drawings.

creasing with increasing temperature. Generally, times as low as 1 hour are adequate at the high temperatures. Suitable environments for use during the final anneal include hydrogen having adeW point no higher than about 80 F. and vacuum, pressures no higher than 1 1O- mm. of mercury being suitable. Lower pressures are especially advantageous, for example, pressures on the order of o l- Further information regarding suitable methods for producing high-purity cube-on-edge grain oriented silicon-iron bodies can be obtained from the pending application of John L. Walter, Serial No. 59, filed'lanuary 4, 1960, and assigned to the same assignee as the present application.

To produce bodies of silicon-iron having constant permeability, the cube-on-edge or (110) [001] grain oriented material is subjected to another anneal at temperatures ranging from about 1000 to 1350 C. for times of not less than about one-quarter hour. This anneal iscarried out in such a way that the bodies are subjected to the presence of a minor amount of oxygen, the oxygen being felt responsible for the achievement of the unique magnetic properties. A dry hydrogen atmosphere can be used for this anneal and oxygen introduced to accomplish the desired properties. An alumina supporting plate has been found etfective in supplying the required oxygen, as later shown. At the termination of this anneal, the samples are cooled at the rate ranging from about 50 to 200 C. per hour.

As an example of bodies produced according to the present invention, an initial body of 0.012 inch thickness was produced according to the procedure outlined earlier, and after the final reduction to 0.012 inch, was annealed at 1200 C. in a vacuum at a pressure of about 6X10- mm. of mercury for 6 hours. This heat treatment caused the samples to orient in the (110) [001] crystalline orientation as evidenced by the hysteresis loops 10 and The D.-C. propertiesof the samples corresponding to curves 10 and 11 are listed in Table I below:

It should be noted that the coercive force of the material at this stage is quite low, while the residual induction and the induction in fields of /2, l and 2 oersteds are comparatively high.

Following the production of the grain oriented bodies, the samples were then placed on a high-purity alumina supporting plate and given an additional anneal in dry hydrogen (dew point -60 F.) at 1200 C. for one hour and then cooled at the rate of 100 C. per hour to about 200 C. These samples are the same ones which were used to obtain the hystersis loops shown in FIG. 1. The specimens, upon once again being subjected to applied magnetic fields, had magnetic properties indicated by curves 12 and 13 in FIG. 2 and listed in Table II followmg:

It should be noted that the coercive force is substantially unchanged, while the residual induction and the inductions measured in fields of "/2 and 1 oersted have 4 been decreased materially and the induction in the 2- oersted field decreased slightly less. Thus, the effect is one of shearing the loop and delivering permeability which is substantially constant from low applied magnetic fields up to those fields where induction approaches saturation.

The development of the sheared loop or constant permeability which is so unique to this alloy is believed caused by the diffusion of minor amounts of oxygen into the alloy body during the final anneal. In the example just given, it is believed that there is a reaction between the hydrogen of the annealing atmosphere and the alumina plates which either produces a small amount of oxygen or a small amount of water vapor which, when contacting the surface of the silicon-iron, decomposes into oxygen and hydrogen. The oxygen then diffuses into the body of the material, causing the change in magnetic properties. It is believed that the oxygen can be supplied by means other than alumina plate as, for example, by providing a small amount of free oxygen in the hydrogen atmosphere during annealing. Test results indicate that if the material is annealed in dry hydrogen alone, no change in the magnetic properties is obtained. Similarly, the properties remain unchanged if the material is placed on plates of alumina and annealed in an argon atmosphere at the same temperature.

In an eifort to determine the means by which the con stant permeability is obtained and specifically to determine whether the etfect was caused by an oxide layer at the surface of the alloy specimen which exerted compressive or tensile stresses on the material, the samples were electropolished to remove about 1 mil from the over-all thickness of the specimens. FIG. 3 of the drawings shows D.-C. hysteresis loops obtained from the same specimens used to produce the FIGURES of 1 and 2 and it is readily apparent that the sheared loop or constant permeability properties are still present. In fact, while the properties of the sample indicated by curve 14 are very similar to the properties indicated by curves 12 and 13, there seems to have been some improvement in the constancy of the permeability of the sample whose properties are indicated by curve 15. The coercive force and other magnetic properties of the samples following electropolishing are contained in Table III.

It is thus apparent that the present invention provides a novel process and magnetic body having properties which have not previously been obtained in iron-base silicon alloys. The constant permeability obtained in this comparatively common and relatively cheap material will have many uses in those applications where heretofore only expensive specialty alloys have been found adaptable.

What we claim as new and desire to secure as Letters Patent of the United States is:

1. In the method for producing cube-on-edge oriented silicon-iron alloy bodies of up to 0.015 inch thickness consisting of from 4 to 6 weight per cent silicon, remainder substantially all iron, and containing not more than about 0.010 weight per cent incidental impurities having a substantially constant magnetic permeability, the steps comprising preparing cube-on-edge oriented bodies of up to 0.015 inch thickness, and heating said bodies in dry hydrogen at from 1100 to 1350 C. for not less than about one-quarter hour in the presence of a small percentage of oxygen to develop substantially constant magnetic permeability.

2. In the method for producing cube-on-edge oriented silicon-iron alloy bodies of up to 0.015 inch thickness consisting of from 4 to 6 weight percent silicon, re mainder substantially all iron, and containing not more than about 0.010 weight percent incidental impurities having a substantially constant magnetic permeability, the steps comprising preparing cube-on-edge oriented bodies of up to 0.015 inch thickness, placing the bodies on high-purity alumina supporting means, and heating said bodies in dry hydrogen at from 1100 to 1350 C. for

not less than about one-quarter hour in the presence of a small percentage of oxygen to develop substantially constant magnetic permeability.

References Cited by the Examiner UNITED STATES PATENTS 2,992,952 7/61 Assmus et a1. 148-111 3,105,782 10/63 Walter 148-113 DAVID L. RECK, Primary Examiner. 

1. IN THE METHOD FOR PRODUCING CUBE-ON-EDGE ORIENTED SILICON-IRON ALLOY BODIES OF UP TO 0.015 INCH THICKNESS CONSISTING OF FROM 4 TO 6 WEIGHT PERCENT SILICON, REMAINDER SUBSTANTIALLY ALL IRON, AND CONTAINING NOT MORE THAN ABOUT 0.010 WEIGHT PER CENT INCIDENTAL IMPURITIES HAVING A SUBSTANTIALLY CONSTANT MAGNETIC PERMEABLILITY, THE STEPS COMPRISING PREPARING CUBE-ON-EDGE ORIENTED BODIES OF UP TO 0.015 INCH THICKNESS, AND HEATING SAID BODIES IN DRY HYDROGEN AT FROM 1100 TO 1350*C. FOR NOT LESS THAN ABOUT ONE-QUARTER HOUR IN THE PRESENCE OF A SMALL PERCENTAGE OF OXYGEN TO DEVELOP SUBSTANTIALLY CONSTANT MAGNETIC PERMEABILITY. 